One-electron reduction of 2- and 6-methyl-1,4-naphthoquinone bioreductive alkylating agents

The semiquinones, Q.-, of derivatives of 2- and 6-methyl-1,4-naphthoquinones, some incorporating leaving groups with substituents such as CH2Br or CH2OCONHCH3, have been produced by radiolytic reduction of Q by (CH3)2COH radicals. The absorption spectra and decay kinetics of Q.- were all closely similar to that produced from 2-methyl-1,4-naphthoquinone, with no evidence for unimolecular elimination of a leaving group in the semiquinone form, but immediate loss of leaving group upon two-electron reduction of Q to the hydroquinone. The redox equilibria between Q/Q.- and O2/O2.- were characterized, and reduction potentials of the couples Q/Q.- in water at pH 7.6 were calculated. The implications of these observations for the use of these compounds as bioreductive alkylating agents or as radiosensitizers with potential selective activity toward hypoxic cells are discussed.